The present invention is directed generally to a collision warning system and method for a motor vehicle.
Modern road vehicles, particularly commercial vehicles, are increasingly being equipped with cruise control systems which enable a vehicle so equipped to achieve and maintain a constant cruising speed, which can be preset by the driver. The preset speed is maintained even when the vehicle is ascending or descending an incline. For this purpose, such cruise control systems are capable of opening or closing the vehicle throttle, as appropriate, or of actuating the vehicle retarder.
Adaptive cruise control (“ACC”) or distance control systems can also measure the distance between a vehicle so equipped and vehicles traveling ahead, and maintain that distance constant. For this purpose, a distance measuring sensor is employed which can be designed as a RADAR or LASER sensor, for example.
When the distance to the vehicle traveling ahead has decreased to a preset index distance, as recognized by the distance sensor, the ACC system readjusts the vehicle distance to the index value by throttling back or by additional actuation of the vehicle service brakes. For trucks, this predetermined distance is typically 50 meters. The vehicle speed may then be lower than the set cruising speed. If the vehicle traveling ahead accelerates beyond the index distance, the set cruising speed is restored. For safety and comfort reasons, the service brake is typically actuated by the ACC system only up to a vehicle deceleration of approximately 2.5 m/s2, which is well below the maximum deceleration of 6 to 8 m/s2.
A critical condition for an ACC system exists when the vehicle traveling ahead brakes suddenly and heavily, or when a vehicle from a neighboring lane moves in front of the vehicle using the ACC system closer than a safe distance. A known method of preventing accidents in such situations is to generate a suitable warning (“system limit warning”) to the driver. Thus, if the braking function of the ACC system is not sufficient, the driver can assist by additional braking in order to maintain a safe distance to the vehicle traveling ahead.
Regardless of whether an ACC system is being used, however, the driver can also be warned if the danger of a rear-end collision is recognized (“collision warning”).
U.S. Pat. No. 5,931,547 describes a vehicle anti-collision system that generates an audible warning for the driver when a time-to-collision (“TTC”) with an object (such as a second vehicle) ahead of the subject vehicle is shorter than a predetermined warning time. In this case, the hydraulic braking system of the subject vehicle is automatically actuated. The distance to the object ahead is determined with a LASER ranging instrument. Automatic braking takes place only if any brake pressure already being applied in the subject vehicle by the driver is smaller than a fixed threshold. That is, automatic braking takes place only when the driver is not braking with enough force.
According to the system described in U.S. Pat. No. 5,931,547, the TTC can be calculated according to the following formula:
  TTC  =      Distance          Δ      ⁢                          ⁢      v      
where:                Δv represents the difference between the speeds of the subject vehicle and the object (second vehicle).        
A warning is transmitted as soon as the TTC becomes shorter than a threshold value.
The system described in U.S. Pat. No. 5,931,547 has the disadvantage, however, that the practical calculation yields a very long TTC, especially at short distances and at a relative speed close to zero. As a result, the situation in these cases is evaluated as non-hazardous by the ACC system, since the TTC is still above the threshold value, even though a hazardous situation actually exists because of the short distance. In addition, the calculation according to the foregoing formula must also be regarded as impractical in highly dynamic driving situations, in which the distance and relative speeds of the vehicles are constantly undergoing rapid changes.
European Patent 0827860 B1 describes a method for adaptive cruise control of a vehicle, as well as a device for implementing the method, wherein a warning is transmitted to the driver if the distance between the subject vehicle and a vehicle traveling ahead becomes shorter than an index value. This index value depends on the driving speed of the subject vehicle, on the weather conditions and on the driving style of the driver. By means of the warning signal, the driver is prompted to brake the subject vehicle manually in addition to the braking applied by the ACC system. The driving response of the driver and driving style are determined from the driver's acceleration and braking actions up to that time.
U.S. Pat. No. 6,604,042 describes a vehicle braking control system in which automatic braking can be initiated when the subject vehicle approaches an object (second vehicle) located ahead. The system described in U.S. Pat. No. 6,604,042 additionally recognizes whether the driver intends to overtake the vehicle traveling ahead. In this case, the automatic braking that would be initiated before any braking by the driver is prevented. A target deceleration is calculated for the subject vehicle that would be necessary to avoid a collision with the vehicle traveling ahead. The calculated target deceleration is then compared against a deceleration threshold of the subject vehicle in a comparator stage. Automatic braking of the subject vehicle is then initiated when, among other circumstances, the calculated target deceleration is greater than or equal to a preset threshold value and further conditions are simultaneously satisfied.
The system described in U.S. Pat. No. 6,604,042 does not provide for possible additional braking by the driver and also does not allow for the driver's reaction time. Nor is a necessary remaining distance to the vehicle traveling ahead taken into consideration. Furthermore, no collision warning signal independent of the control action is generated for the driver.
Accordingly, it is desired to provide an improved collision warning system and method for a motor vehicle that avoids the foregoing disadvantages and that are capable of generating a reliable warning signal for the driver if it is anticipated that a remaining distance to a vehicle traveling ahead cannot be maintained at the end of ACC system braking or after braking by the driver.